BLOOD SYSTEM. , j t 



mon arrangement, but there is some variation in this 

 regard. 



Breathing. — If we watch a hornet or wasp at rest, 

 we will observe a back-and-forth movement, an alter- 

 nate lengthening and shortening, of the abdomen. This 

 enlargement and contraction of the body cavity draws 

 in and expels air. We at once, therefore, see why oil is 

 so fatal to insects. It covers the spiracles with a film and 

 thus suffocates the insect. 



SECTION V. 

 Lymphatic or Absorbent System. 



Besides the blood system, there is another system of 

 vessels penetrating the tissues everywhere, which may 

 be regarded as supplementary to the blood system. It 

 carries not blood, but a clear liquid called lymph, and is 

 therefore called the lymphatic system. It is not a circu- 

 latory system, but purely an absorbent or drainage sys- 

 tem. We have already seen those of the intestines — 

 viz., the lacteals (page 326) ; but they are not confined 

 to the intestines, but occur everywhere. Nor is their 

 function even in the intestines confined to the absorp- 

 tion of food ; they absorb many other things. They are 

 far less understood, both as to their distribution and as 

 to function, than the blood system, for they are diffi- 

 cult to see, as they carry colorless liquid; and they are 

 difficult to inject on account of their valvular structure. 

 We treat them, therefore, very briefly. 



General Description. — They begin by blind ex- 

 tremities in all the tissues, but especially in the ab- 

 dominal viscera, forming a capillary network (Fig. 290), 

 and therefore increasing but little in size until they 

 reach the larger trunks. The great emptying trunks are 

 (1) the thoracic duct on the left side of the backbone, 



